Field-effect tubular transistor



United States Patent O 3,153fl52 FHELD-EFFECT TUBULAR TRANSHTGR Stanislas Teszner, 49 Rue de la Tons, Paris, France Filed Feb. 10, 1951, Ser. No. 83,417 Ciainls priority, appiieation 'rance, Feb. 13, 1960,

54 3 Claims. (Cl. Sl-4.35)

The present invention concerns field-effect tubular transistors having a hollowed-out throat in the outside surface between the ends of the tubular transistor body.

Field-effect semi-conductor devices are known in the art -and amongst these semiconductor devices are those Which are known as tecnetrons, in which a conductive channel is constituted by carriers fiowing in a cylindrical semiconductive rod between a source electrode and a drain electrode and is acted upon by a gate electrode shaped like a circular ring encircling the rod. These tecnetrons are field-effect centripetal pinch-off transistors in the sense that if a potential is applied between the gate electrode and the semiconductive rod in the reverse direction and a space charge region is thus created in the semiconductive rod, the field developed by this potential in the space charge region is a radial direction field.

The object of the invention is to make modified semiconductor devices based upon the tecnetron, Which are capable of being utilized as power elements (dissipation power ranging from one to several tens of watts) at very high frequencies.

The field-eifect semiconductor device of the invention includes a tubular stub or body made with a semiconductive material, a circular throat hollowed out in the outside surface of the tubular stub, between the ends thereof, at least one annular metallic electrode placed in the throat, having a rectifying contact with the semiconductor and having an axial extension lower than that of the throat, ohrnic electrodes on the terminal surfaces of the tubular stub, said tubular stub comprising on the part of its inside surface which is opposite at the bottom of the throat an inversion layer, such as an oxide layer formed as an oxide of the semiconductive material whose type of conductivity is difierent in kind from that of the semiconductive material constituting the tubular stub. The thickness of this inversion layer formed of oxide of semiconductive material of the tubular body is at least about 100 angstrorn units.

Semiconductor devices are known which utilize the field-elfect phenornenon in a tubular configuration according to German Patent No. 920,971. However, up to the present time it has been considered necessary to produce this effect simultaneously from two concentric annular metallic electrodes, the one exerting a centripetal pinch-off and the other a centrifugal pinch-oif. The manufacture of the structure of this German Patent No. 920,971 is particularly diflicult to carry out, especially the manufacture of the internal electrode, and it has not been possible to develop these devices for their use in industry.

However, the applicant, working from theoretical conceptions, described a tubular device for the first time in a report to the Socit Francaise des Electriciens, on April 16, 1948, published in the August 1949 Bulletin of this Society and developed this description further in the June 1954 Revue Gnrale de 1Electricit, (pages 319- 334) whereafter the tubular transistor device was able to be manufactured by a relatively simple new process a practical, commercially usable tecnetron being realized. This manufactured tecnetron device was a field-eifect transistor having a tubular structure made up of only one outer electrode and the inner surface of the tube opposite this electrode being processed in a novel but simple chemical process whereby the semiconductive material is con- "ice verted in situ to the oxide at the surface thereby forming an inversion layer without the necessity of providing a metallic electrode.

lndeed, the outcome of the researches mentioned above (see, also, Progress in Semiconductors, vol. 5, Heywood Company Ltd., London, 1960, page 4), is that the semiconductors comprise, in certain cases, electrons in the surface states i.e. an electron layer in the case of a n type semiconductor and holes in the surface states i.e. a hole layer in the case of a p type semiconductor and this layer for each case is called an inversion layer, which is progressively balanced by a space charge of the opposite sign, extended in depth. However, in order that there form the just-described double layer on and near the surface of the semiconducotr, it is absolutely necessary that this surface receives a special cleaning and oxidizing treatment which will be described in detail, hereinafter.

It is comparatively easy to form, in situ, the inversion layer on the inner surface of the tubular lstub in accordance with the novel method which I have discovered for use with unipolar field effect transistors.

As it is equally easy to produce a barrier layer on the outer surface whose thickness can be modulated by means of tan :annular metallic electrode having a rectifying contact with the semiconductor, the tubular tecnetrons of the invention can be easily manufactured.

A tubular tecnetron offers, in comparison With a conventional tecnetron, certain drawbacks resulting, in particular, from the fact that the section of the tubular body is limited to an area close to the outer circumference; in consequence, although the pinch-off is centripetal, the average variation rate of the conductive channel section with the potential applied to the gate electrode is, in this case, much smaller than in that of a full cylindrical configuration for the *same transversal dimension. Consequently, for the same pinch-olf potential, the transverse dimension would have to be much smaller and the variation rate of the conductance with the potential will also be reduced. On the other hand, this device offers the appreciable advantage of a notably higher power limit for a single unit, without any considerable complications in the manufacture, all the While maintaining a remarkably operation frequency limit. Typical numerical values of electrical Characteristics will be given in the examples.

The invention will now be described Kin detail in connection with the appended drawings in which:

FIGURES 1 and 2 show the distribution of the space charges in a serniconductor tubular stub axial section provided with a barrier layer electrode on its two opposite faces, or of ia barrier layer electrode on one face only, the second face remaining bare and nonprocessed;

FIGURES 3 and 4 show the distribution of the space charges in the case of a serniconductor tubular body having a barrier layer electrode in the throat on the outside of the tubular body and an inversion layer formed in situ and consisting of the omde of the semiconductor material of the tubular body, this oxide layer being located on the opposite face to the barrier electrode, e.g., on the inner surface of the tube;

FIGURES 5, 6 and 7 represent respectively tubular tecnetrons with one or several control electrodes and a tubular tecnetron provided With ra base;

FIGURE 8 represents, schematically, the apparatus for the hollowing out of a counter-throat on the inner face and the treatment of this inner surface;

FIGURES 9 and 10 represent the power tubular tecnetron placed in a holder.

Referring now to FIG. 1, 1 designates an xx' axis semiconductor tubular stub section (section limited to one side of the axis), provided on its two faces With aisavea rectifying contact electrodes 2 and 3, that is to say with the barrier layer electrodes. The two barrier layer electrodes are interconnected by connection lead 53. FIG- URE 1 shows the distribution of the space charges in the case of a n type semiconductor when there is applied between the electrodes on the one hand land the semiconductor on the other hand a potential difference of a sign corresponding to the barrier layers solicitation in the reverse d-irection and of a suflicient amplitude to produce a complete pinch-oif of the conductive channel. According to the mechanism which is explained in the applicant's article published in the October 1958 Bulletin de la Socit Francaise des Electriciens, pages 683-698, there now occuns a complete elimination of the free carriers, due [to the negative charges clinging to the surface against the electrodes and the positive charge-s being distributed in the semiconductive section.

yFIGURE 2 represents an xx' axis semiconductor tribular stub 1 section provided with a barrier layer electrode 2 on only one of its faces, the other face being left bare; without special treatment. Thus, as the negative charges do not cling to this face, free carriers will be available and, in other words, a surface layer 4 will remain conductive whatever the difference of potential applied between electrode 2; and the semiconductor. Layer 4 can vary in thickness (the dimensions indicated on the figure have been given solely as an example) Without achieving a complete pinch-oif of the conductive channel.

FIGURES 3 and 4 represent a semiconductive tubular stub 6 section comprising on one side a rectifying contact electrode 2 and, on the other side, a bare face 5 which has received a treatment for the production of an inversion layer allowing surface clinging of the negative charges whereby there can be achieved the possibility of space charge extension in the whole of the section and the obtainment of a complete pinch-off of the conductive channel due to the difference of potential applied between electrode 2 and semiconductor 6. The details of the chemical treatment 'for the formation of this inversion layer perfected by the applicant Will be described in the examples which appear later in the application.

However, in order that the rectifying electrode inside the tubular stub can be eliminated, the remaining electrode must form a Faraday cage of the conductive channel whose conductance is to be modulated. Such is the case for the tubu-lar tecnetron of the invention whose outer annular electrode 2 completely encircles the annular space 6 in Which is developed the conductive channel. FIGURE 5 shows, in perspective, a tubular tecnetron. It is made up of ra tubular body 8 in semiconductive material, of n-type or p-type, in germanium, silicon or in a combination of inter-metallic elements of Groups III and V of the Periodic Table. This tubular body is obtained from a monocrystal, for example by, the conventional ultra-sonic cut-ting process, and is thereafter formed with a throat 16 separating the tu'bular body into two end portions 9 and 9'. This throat is formed by taking material away ifrom the tubular body, eg., the body is hollowed out by the electrolytic process described in detail in U.S. application Ser. No. 764,105 filed September 29, 1958, in the name of the present applican-t.

The shaping of the throat portion in the tubular body is obtained by impinging electrolyte jet on the outer surface of the tubular body while the latter is uniformly rotated about its longitudinal axis. For a tubular body diameter, for example, ranging from 3 to 20 mm. (from IAs" to sl/f), it is best to utilize a nozzle projecting a comparatively wide jet whose thickness Will be about the throat width. With this in view, the nozzle has a reotangular or oval section. The t-hroat shaping operation is thus efectively controlled by the process described in the above patent application. An annular metallic electrode lll is placed in throat iii and it has a lower axial width than the axial width at the bottom of the throat in order to define between the ring edges and the throat sides, metal free side hands in the throat 7 and 7' which strongly reduce the capacity existing between the electrode and the part of the conductive channel located in the Itubular end portions 9 and 9'. The metal electrode lll is, in a preferred example, formed of electroplated indium. For the plating of this electrode, which is carried out immediately after the electrolytic jet shap-ing, -a circular section nozzle is preferably employed and the nozzle diameter must be less than the width (in the axial direction) of the throat bottom. The -tu-bular body inner surface remains bare up to this point in the method and then undergoes a treatment to produce an oxide layer which provides an inversion layer, the details of which Will be given further on. Finally, the tubular body is provided With ohmic terminal electrodes 12 and 13 Shaped in the form of rings or welded metallic plates, at least one yof the two electrodes being a ring in order to allow access into the inside of the tubular body as is explained in relation to FIG. 8. Source 15, gate 16 and drain 14 connections are Secured respectively to electrodes 13, 11 and 12.

FIGURE 6 shows, in perspective, an alternative embodiment in which the tubu-lar body 17 includes a throat 18 at the bottom of which are found, no longer one control electrode, but two electrodes 19 and 29 according to the principle set out in U.S. Patent 2,921,i265 issued January 12, 1960. Here there are four electrodes 19, 20, 21 and 22 and .four connections 23, 24, 25 and 26. In this configuration ring 19 is least w-ide and therefore exhibits ig-reatly reduced capacity with respect to the conductive channel and ensures the section modulation of this conductive channel. The widest ring is placed upstream in the fiux direction of the majority carriers. In the case considered for a structure comprising an n-type semiconductor, these majority oarriers are electrons and source electrode 21 then constitutes the cathode and drain 22 electrode constitutes the anode and the elements coact as an electrostatic lens. vA particularly high operative frequency limit is thus obtained and quantitative values are shown in a later part of this description. The number of control electrodes can be raised to three as explained in US. Patent 2,921,265 mentioned above.

-FIGURE 7 shows, in perspective, an alternate embodiment comprising, in addition to tubular body 27 which is shortened, a semiconductor base 28 for the purpose of increasing the strength and to reduce the stray series-resistance between the .gate electrode 31 and one of the terminal electrodes of the device. The electrode n welded to this semiconductive base 23, constitutes the source electrode and electrode 29 constitutes the drain electrode. It is between the source electrode and the gate electrode that it is essential to reduce the series resistance .to a

The essential treatment of the tubular stub inner surface opposite throat lll of FIG. 5, or 13 of FTG. 6, or 32 of FIG. 7, includes two steps. The first step is a treatment in a very dilute potash bath containing for example l gram of potassium hydroxide per liter of water.

During the first step, the apparatus shown schematically in FiG. 8 is utilized. Tube 9 provided with its throat lill, with its electrodes lll, 12 and 13 and its connections 14-, 15 and 16, is placed in a tank 36 filled with the dilute potash solution. Inside the tank and in the plane containing the gate electrode lll, a metallic ring 39 outside tube 9 and a metallic disk 4G inside the aforesaid tube are arranged. The means of supporting ring 39 and disk 49 contain the current feed wires but are insulated on the outside, the insulated layer protecting against corrosion by the potash. Connections 14 and 15 can be joined direetly to the positive terminal of the current source 33 or be connected to this terminal pole through two equal resistors 37 and 37' whose common point is then connected to this terminal. These two resistances 37 and 37' allow the passage of an alternating current produced by a constant current source7 represented by the alternating current source 34 and the impedance 58 in series, in-

A), the connections are those represented in FIG. 8 at the letter A. Ring 39 and disk 40 are at the negative potential and tubular stub 9 is at the positive potential. The gate electrode is at the negative potential so as not to be etched.

The following sub-step is a hollowing out stage of the counter-throat (sub-step B), the connections being those shown in FIG. 8 at the letter B. Ring 39 is no longer connected to the current source. Disk 40 and tube 9 are connected to the current source as in stage A. The gate electrode is connected this time to the positive terminal through resistance 41, this resistance being regulated so that the gate electrode potential is positive (in the case of a semiconductor body of the n-type) of a few tenths of a volt (0.3 volt, for example) with respect to zone a in the course of hollowing out. The common point of the resistances 37 and 37' is connected to the positive terminal and an alternative current passes longitudinally through tube 9. The gate electrode being biased positively with respect to the semiconductive body, minority carriers are injected and go towards region a Where they reduce the potential barrier between the electrolyte and the semiconductive body, thus allowing the localization of the etching in region a.

The following sub-step (sub-step C) is a sub-step of cleaning the gate electrode edges. Ring 39 and disk 40 are not connected to the current source, electrode 12 is connected to the positive terminal and the gate electrode 11 is connected to the negative terminal. This electrode is then biased in the reverse direction. Etching takes place then on either side of the gate electrode, which cleans the parts 7 and 7' (FIG.

Finally, in a last sub-step with potash (sub-step D) a new general cleaning is carried out the connections being the same as in sub-step A.

Then comes a washing step in distilled water followed by an imrnersion in strong boiling peroxide (from 30 to 35%, for example) or at least heated to the point of provoking a notable release of nascent oxygen. A perfectly polished deeply oxidized surface is thus obtained and the forming of an inversion layer is ensured, the oxygen atoms fixing the electrons on the surface. This latter process is continued until the oxide layer has a thickness of at least 100 angstroms. This corresponds for germanium to about two minutes time interval.

The tecnetron thus treated is then partially embedded in a synthetic resin, for example in a phenolic resin or in a silicone base resin, as shown in 48 in FIG. 9. It is then welded on base 46. This weld establishes conduction between electrode 13 and base 46. On the other hand connections 14 and 16 are welded respectively to the outlet plugs 43 and 44 which pass through base 46 by insulated pinches 42 and 42'; the third outlet is plug 45 welded directly to the base. Shield 47 is welded electrically for example onto the base circumference. FIG. shows the plan view of the FIG. 9 device, the shield being removed.

The chief geometric features of the device are, for guidance, the following in the case of the germanium:

Outer diameter of tubular stub: from 3 to 20 mm. (approximately) Inner diameter of tubular stub respectively: from 1 to 19 mm. (approximately) 6 Tube thickness before electrochemical treatment: 600yl Height of tube: l to 3 mm. (%4, to 1/8) Width of gate throat: to 500a Width of gate electrode: 50 to 400m Wall thickness at gate spot: 20 to /r The electrical Characteristics are for guidance, the following for germanium.

(a) For device with only one gate electrode:

Cperative frequency limit ln large bandwidth amplifier, about 100 mc./s. In narrow bandwidth amplifier, about 200 mc./s. T ransconductance: a few milliamperes per volt (about 3 to 5 m./v.); however it can be increased practically at will by putting units in parallel Dissipation power: at least about 20 watts, but it can, also, be correlatively increased by putting units in parallel Amplication factor: .at least 50 (b) For devices of two or three gate electrodes, the operative frequency limit is raised perceptibly beyond 1,000 mc./s., the dissipation power limit remaining the same as above.

What I claim is:

1. A field-effect centripetal pinch-off semiconductor device comprising a tubular body consisting of a semiconductor material, a hollowed-out throat formed between the ends of the body in the outer surface of and extending continuously around said body, said hollowed throat having a predetermined axial length, a metallic gate electrode in said hollowed throat making a rectifying contact with said tubular body and having an axial length which is less than the axial length of said hollowed throat, metallic electrodes on the annular end faces of said tubular body which makes ohmic contact thereto and a saturated oxide layer on the interior surface of the tubular body along a surface which is opposite said throat.

2. A field-effect centripetal pinch-olf semiconductor device comprising a tubular body consisting of a serniconductive metal, a hollowed-out throat formed between the ends of the body in the outer surface of and eXtending continuously around said body, said hollowed throat having a predetermined axial length, a metallic gate electrode in said hollowed throat making a rectifying contact with said body and having an axial length which is less than the axial length of said hollowed throat, metallic electrodes on the annular end faces of said tubular body which makes ohmic contact thereto and an oxide layer constituted by a layer of the saturated and dehydrated oxide of said metal on the interior surface of the tubular body along a surface which is opposite said throat.

3. A field-effect centripetal pinch-of semiconductor device comprising a tubular body consisting of a semiconductive metal, a hollowed-out throat formed between the ends of the body in the outer surface of and extending continuously around said body, said hollowed throat having a predetermined axial length, a metallic gate electrode in said hollowed throat making a rectifying contact with said tubular body and having an axial length which is less than the axial length of said hollowed throat, metallic electrodes on the annular end faces of said tubular body which makes ohmic contact thereto and an oxide layer constituted by a layer of the saturated and dehydrated oxide of said metal having a thickness of at least 100 A. on the interior surface of the tubular body along a surface which is opposite said throat.

References Cited in the file of this patent UNITED STATES PATENTS 2,754,455 Pankove July 10, 1956 2,987,659 Teszner June 6, 1961 3,007,119 Barditch Oct. 31, 1961 

1. A FIELD-EFFECT CENTRIPETAL PINCH-OFF SEMICONDUCTOR DEVICE COMPRISING A TUBULAR BODY CONSISTING OF A SEMICONDUCTOR MATERIAL, A HOLLOWED-OUT THROAT FORMED BETWEEN THE ENDS OF THE BODY IN THE OUTER SURFACE OF AND EXTENDING CONTINUOUSLY AROUND SAID BODY, SAID HOLLOWED THROAT HAVING A PREDETERMINED AXIAL LENGTH, A METALLIC GATE ELECTRODE IN SAID HOLLOWED THROAT MAKING A RECTIFYING CONTACT WITH SAID TUBULAR BODY AND HAVING AN AXIAL LENGTH WHICH IS LESS THAN THE AXIAL LENGTH OF SAID HOLLOWED THROAT, METALLIC ELECTRODES ON THE ANNULAR END FACES OF SAID TUBULAR BODY WHICH MAKES OHMIC CONTACT THERETO AND A SATURATED OXIDE LAYER ON THE INTERIOR SURFACE OF THE TUBULAR BODY ALONG A SURFACE WHICH IS OPPOSITE SAID THROAT. 